Hóa học - Chapter 14: Ethers, epoxides, and sulfides

An epoxide is higher in energy than an acyclic ether by about 25 kcal/mol ring strain. Release of the ring strain makes the opening of an epoxide thermodynamically favored. Strong bases, such as Grignards and organolithiums, open the epoxide ring by attacking the less hindered carbon.

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Chapter 14ã 2010, Prentice HallOrganic Chemistry, 7th Edition L. G. Wade, Jr.Ethers, Epoxides, and SulfidesChapter 14*EthersFormula is R—O—R¢where R and R¢ are alkyl or aryl.Symmetrical or unsymmetricalOCH3CH3OCH3Chapter 14*Structure and PolarityOxygen is sp3 hybridized.Bent molecular geometry.C—O—C angles is 110°.Polar C—O bonds.Dipole moment of 1.3 D.Chapter 14*Boiling PointsSimilar to alkanes of comparable molecular weight.Chapter 14*Hydrogen Bond AcceptorEthers cannot hydrogen-bond with other ether molecules. Molecules that cannot hydrogen-bond intermolecularly have a lower boiling point. Ether molecules can hydrogen-bond with water and alcohol molecules.Chapter 14*Solvation of Ions with EtherAn ionic substance such as lithium iodide is moderately soluble in ethers because the small lithium cation is strongly solvated by the ether’s lone pairs of electrons. Unlike alcohols, ether cannot serve as hydrogen-bond donors, so they do not solvate small anions well.Chapter 14*Ether ComplexesGrignard reagents: Complexation of an ether with a Grignard reagent stabilizes the reagent and helps keep it in solution. Electrophiles: The ethers nonbonding electrons stabilize the borane (BH3).Chapter 14*Crown Ether ComplexesCrown ethers can complex metal cations inside the ring. The size of the cation will determine the size of the ring needed. Complexation by crown ethers often allows polar inorganic salts to dissolve in nonpolar organic solvents.Chapter 14*Common Names of EthersName the two alkyl groups attached to the oxygen and add the word ether.Name the groups in alphabetical orderSymmetrical: Use dialkyl or just alkyl.diethyl ether orethyl ethert-butyl methyl ether ormethyl t-butyl ether CH3CH2OCH2CH3CH3OCCH3CH3CH3Chapter 14*IUPAC NamesThe more complex alkyl group is the alkane name. The small group (with the oxygen) becomes an “alkoxy” group.2-methyl-2-methoxypropaneMethoxycyclohexane CH3OCCH3CH3CH3OCH3Chapter 14*Cyclic EthersHeterocyclic: Oxygen is part of the ring. Epoxides (oxiranes) Oxetanes Furans (Oxolanes ) Pyrans (Oxanes )DioxanesChapter 14*Epoxide Nomenclature Name the starting alkene and add “oxide”.Hcyclohexene oxideperoxybenzoic acidOHHOH1,2-epoxycyclohexane The oxygen can be treated as a substituent (epoxy) on the compound. Use numbers to specify position.Chapter 14*Epoxide Nomenclature (Continued) The three-membered oxirane ring is the parent (oxygen is 1, the carbons are 2 and 3). Substituents are named in alphabetical order.trans-2-ethyl-3-methyloxiraneOHHCH3CH3CH2Chapter 14*IR Spectroscopy of EthersIR: The C—O stretch is in the fingerprint region around 1000–1200 cm-1.Many compounds have the C—O stretch.If the IR spectrum has the C—O stretch but does not have a C═O or an OH stretch, then the compound is most likely an ether.Chapter 14*MS Spectrometry of EthersMain fragmentation is the -cleavage to form the resonance-stabilized oxonium ion.Either alkyl group can be cleaved this way.Chapter 14*Loss of an Alkyl GroupThe C—O bond can be cleaved to produce a carbocation.Chapter 14*MS Spectra of Diethyl EtherChapter 14*NMR Spectroscopy of EthersThe typical chemical shift for ethers in NMR are: 13C—O signal between  65–90. 1H—C—O signal between  3.5–4. Chapter 14*Williamson Ether SynthesisThis method involves an SN2 attack of the alkoxide on an unhindered primary halide or tosylate. Chapter 14*Examples of the Williamson SynthesisChapter 14*Phenyl EthersPhenoxide ions are easily produced for because the alcohol proton is acidic.Phenyl halides or tosylates cannot be used in this synthesis method.OH+NaOHO_Na++HOHChapter 14*Why is the following reaction a poor method for the synthesis of t-butyl propyl ether?What would be the major product from this reaction?Propose a better synthesis of t-butyl propyl ether.The desired SN2 reaction cannot occur on the tertiary alkyl halide.The alkoxide ion is a strong base as well as a nucleophile, and elimination prevails.Solved Problem 1SolutionChapter 14*(c) A better synthesis would use the less hindered alkyl group as the SN2 substrate and the alkoxide of the more hindered alkyl group.Solved Problem 1 (Continued)Solution (Continued)Chapter 14*Alkoxymercuration–Demercuration ReactionUse mercuric acetate with an alcohol. The alcohol will react with the intermediate mercurinium ion by attacking the more substituted carbon. Chapter 14*Industrial Ether SynthesisIndustrial method, not good lab synthesis.If temperature is too high, alkene forms.140°CHOCH2CH3CH3CH2OHCH3CH2OCH2CH3+H2SO4Chapter 14*Cleavage of EthersEthers are unreactive, which makes them ideal solvents for a lot of different reactions.They can be cleaved by heating with HBr and HI.Reactivity: HI > HBr Chapter 14*Mechanism of Ether Cleavage The acidic conditions will protonate the oxygen.The halide will attack the carbon and displace the alcohol (SN2).The alcohol reacts with the acid to form more alkyl halide. This last step will not occur with phenol.Chapter 14*Phenyl Ether CleavagePhenol cannot react further to become halide because an SN2 reaction cannot occur on an sp2 carbon.OCH2CH3HBrOH+CH3CH2BrChapter 14*Autoxidation of EthersIn the presence of atmospheric oxygen, ethers slowly oxidize to hydroperoxides and dialkyl peroxides.Both are highly explosive.Precautions:Do not distill to dryness.Store in full bottles with tight caps. Chapter 14*Mechanism of AutoxidationChapter 14*Sulfides (Thioethers)R—S—R, analog of ether.Name sulfides like ethers, replacing “sulfide” for “ether” in common name, or “alkylthio” for “alkoxy” in IUPAC system.methyl phenyl sulfide ormethylthiobenzeneSCH3Chapter 14*Thiols and ThiolatesThiolates are easily synthesized by the Williamson ether synthesis, using dithiolate as the nucleophile.Chapter 14*Sulfide ReactionsSulfides are easily oxidized to sulfoxides and sulfones. Sulfides react with unhindered alkyl halides to give sulfonium salts.CH3SCH3H2O2CH3COOHCH3SCH3OCH3COOHH2O2CH3SCH3OO+CH3SCH3CH3ICH3SCH3CH3+I_Chapter 14*Sulfides as Reducing AgentsBecause sulfides are easily oxidized, they are often used as mild reducing agents.Chapter 14*Synthesis of EpoxidesPeroxyacids are used to convert alkenes to epoxides. Most commonly used peroxyacid is meta-chloroperoxybenzoic acid (MCPBA).The reaction is carried out in an aprotic acid to prevent the opening of the epoxide.Chapter 14*Selectivity of EpoxidationThe most electron-rich double bond reacts faster, making selective epoxidation possible.Chapter 14*Halohydrin CyclizationIf an alkoxide and a halogen are located in the same molecule, the alkoxide may displace a halide ion and form a ring. Treatment of a halohydrin with a base leads to an epoxide through this internal SN2 attack.Chapter 14*Epoxides via HalohydrinsChapter 14*Acid-Catalyzed Opening of EpoxidesAcid-catalyzed hydrolysis of epoxides gives glycols with anti stereochemistry. Anti stereochemistry results from the back-side attack of water on the protonated epoxide.Chapter 14*Acid-Catalyzed Opening of Epoxides in Alcohol SolutionA molecule of alcohol acts as the nucleophile and attacks and opens the epoxide. This reaction produces an alkoxy alcohol with anti stereochemistry. Chapter 14*Base-Catalyzed Opening of EpoxidesThe hydroxide ion attacks and opens the ring.The diol is obtained after protonation of the alkoxide with water. Chapter 14*Ring Opening in BaseAn epoxide is higher in energy than an acyclic ether by about 25 kcal/mol ring strain. Release of the ring strain makes the opening of an epoxide thermodynamically favored.Chapter 14*Regioselectivity of EpoxidationChapter 14*Predict the major products for the reaction of 1-methyl-1,2 epoxycyclopentane with(a) sodium ethoxide in ethanol(b) H2SO4 in ethanolSodium ethoxide attacks the less hindered secondary carbon to give (E)-2-ethoxy1 methylcyclopentanol.(b) Under acidic conditions, the alcohol attacks the more electrophilic tertiary carbon atom of the protonated epoxide. The product is (E)-2 ethoxy-2-methylcyclopentanol.Solved Problem 2SolutionChapter 14*Biosynthesis of SteroidsChapter 14*Reaction of Epoxides with Grignard and OrganolithiumsStrong bases, such as Grignards and organolithiums, open the epoxide ring by attacking the less hindered carbon.

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